What is the difference between var and dynamic in C#?

var is used for implicit typing of local variables. The compiler determines the variable's type at compile time. After the type is determined, it becomes fixed and cannot be changed.

// The compiler determines the type 'int' for the variable number
var number = 10;
// number = "hello"; // Compilation error: Cannot convert string to int

dynamic is used to indicate that member resolution (methods, properties, etc.) for the variable will be performed at runtime, not at compile time. This allows using variables whose type is unknown at compile time.

// The type of the variable obj is determined at runtime
dynamic obj = "hello";
// Calling a method that may or may not exist in the string type
var length = obj.Length;

obj = 10;
// Calling a method that may or may not exist in the int type
// var result = obj.Substring(1); // Runtime error, int does not have a Substring method

Comparison:

Characteristic	var	dynamic
Typing	Strict (determined by the compiler)	Dynamic (determined at runtime)
Member resolution	At compile time (.)	At runtime (.)
Error checking	At compile time	At runtime
Performance	High	Lower, due to overhead of dynamic resolution
Usage	Simplifies syntax when type is obvious	Interacting with dynamic APIs, COM, DLR